1. Field of the Invention
The present invention relates to nonreciprocal circuit devices, and in particular, relates to a nonreciprocal circuit device such as an isolator and a circulator used in a microwave band.
2. Description of the Related Art
Conventional nonreciprocal circuit devices such as isolators and circulators have had characteristics that transmit signals only in a predetermined specific direction and do not transmit signals in the opposite direction. For example, isolators are used in transmitter circuits in mobile communication equipment such as car phones and cellular phones, using the characteristics described above.
A nonreciprocal circuit device of such a type includes an assembly that includes a ferrite in which center electrodes are provided and permanent magnets that apply a direct current magnetic field to the ferrite. An improvement in electric characteristics, a reduction in size, especially, a reduction in profile, and the like are required.
International Publication No. 2007/046299 describes a nonreciprocal circuit device in which a ferrite in which a first center electrode and a second center electrode are provided and permanent magnets are disposed so as to have a shape that has front and back rectangular principal surfaces of the same size, the respective principal surfaces opposing each other so that the respective outer shapes coincide with each other.
However, when the respective outer shapes of the respective principal surfaces of a ferrite 32 and permanent magnets 41 are the same, as shown in FIG. 13A, since leakage flux φ3 occurs at ends, magnetic flux φ2 at the ends is smaller than magnetic flux φ1 at the center. This arrangement has a problem in that the density of magnetic flux applied to the principal surfaces of the ferrite 32 becomes uneven, and thus the insertion loss decreases.
In order to improve the condition, the outer shape of the permanent magnets 41 may be enlarged, as shown in FIG. 13B. In this case, even when the leakage flux φ3 occurs, the magnetic flux φ2 applied to the ends of the ferrite 32 is substantially equivalent to the magnetic flux φ1 at the center. However, in this remedy, since the permanent magnets 41 become larger, the size of the nonreciprocal circuit device is increased. In particular, a reduction in profile is greatly impaired.